Most cars that bring big bucks at auction today were built in low numbers. Combine rarity with special history/engines/whatever, and you have the possibility that it will become a collectible car. As much as owners love their Magnums, the reality is that it was produced in rather large quantities, meaning it probably won't become a sought after car.
But, even rarity and special status can't guarantee big bucks. I know a guy that bought a Ferrari Testarossa in the 80's thinking that the unique looks, high performance and name would have the car worth a ton someday. Sure, examples today sell for good money, but not the massive dollars that truly special and super-rare Ferrari models might fetch. Today, he just drives the snot out of it, as it's worth a lot less than what he paid, and he figures he better enjoy it.
The other problem the Magnum has is the sheer quantity of high hp available in the market. The Buick GNX (VERY low production...) was a killer machine in its day, crushing everything else that was stock, and lots that weren't.
Cars I suspect might become auction queens someday include the super exotics like the Mercedes SLS, Bugatti Veyron and so on. In the lower prices, I'd put a little money on a Corvette ZR-1, depending on final numbers. There's a guy somewhere in my burb that owns "both" ZR-1 Vette models, and his old one is a 100-pt concours car and he could get a pretty good buck for it. Might be worth some real dough someday. Porsche's GT3 machines are pretty rare, but only super-exotic Porsches seem to become big-bucks as many Porsche owners actually drive their cars. That said, a fully restored 550 Spyder, 356C or Speedster can fetch some pretty high prices. Rust helped the owners of those cars! So many just rusted away, it took care of the higher production numbers...
